即時細緻可調性視訊在無線區域網路下之傳輸效率最佳化研究

摘要

    隨著多媒體應用快速普及與多媒體資料編解碼技術成熟，加上無線網路蓬勃發展，多媒體資料在無線區域網路傳輸將是一個不可避免趨勢。當封包在具有高錯誤機率的無線網路傳輸時，封包長度過小將使得封包的檔頭額外負荷提高，封包長度過大亦使封包發生錯誤機率提高，兩者皆會降低頻寬的使用效率。此外，在視訊編碼的過程中，不同型態的視訊封包具有不同重要性，若一視同仁地直接傳輸於無線網路中，將使得多媒體品質遭受嚴重的影響。

     在本論文中，根據上述問題提出一套在無線區域網路下具有延遲限制的視訊封包最佳傳輸機制。本論文首先依據無線網路的傳送現況、封包檔頭的額外負荷、輪詢排程延遲、傳輸延遲、以及使用人數等因素，利用數學分析而得到最佳的封包長度。以此最佳封包長度為基礎，再針對MPEG-4 FGS特性以及視訊封包重要性差異，利用具優先等級的自動重送機制提出一套非對稱式視訊封包保護機制。本論文成果同時適用於所有無線區域網路，並且同時適用於IPv6以及IPv4網路層協定架構。模擬結果顯示，在IPv4/IPv6 IEEE 802.11 b/a/g等六種網路環境中，利用本論文所決定的封包長度傳輸於十種不同錯誤狀況時，均能達到最大頻寬使用率；此外，視訊品質亦有效地提升約1~2 dB。

　

The study of optimal transmission

for real-time FGS videos over WLAN

Abstract

With the technology advances in digital compression and wireless networks, real-time streaming of video content over IEEE 802.11 wireless networks is a multimedia application full of potentials. However, transmission over the error-prone channels by using larger packet size results in higher packet error rate, while transmission by using smaller packet size is inefficient because of the fixed overheads. Both of them reduce the utilization of bandwidth. Meanwhile, IEEE 802.11 wireless networks are relatively unreliable due to an error prone physical channel and the lack of QoS guarantee. Therefore, the video quality transmitting over wireless networks will possibly be degraded.

In the thesis, we propose an optimal transmisission mechanism. First, in order to optimize the utilization of bandwidth, the optimal packet size will be decided by using analyses according to the error situation, transmission overheads, scheduling delay, transmission delay, and number of connections. Base on the optimal size of packets, the priority ARQ in the IEEE 802.11 point coordinator adapts the repeat limits depend on the video packet priorities to achieve both high channel utilization and good video quality. Simulation results show that with ten error patterns and three video sequences, the proposed mechanism indeed achieves maxmum utilization of bandwidth over all IPv6/IPv4 IEEE 802.11b/a/g wireless network. Meanwhile, the proposed mechanisms can improve the PSNR by 1 to 2dB on average under wireless channels with the range of mean burst error length from 1 to 10 bits at the range of bit error rate from 2×10-4 to 8×10-5.